Numerous types of pumps having a plurality of features directed to improving performance of the pump for one or more specific applications are currently available on the market today. One application where pumps have evolved significantly to address the needs of the industry are those which are designed to transport food-related products such as ground meat, emulsifications, stews, etc. When pumps of this type supply such products to packaging or other processing equipment positioned downstream from the pump, it is often important that the pump supply a substantially continuous flow of product at a predetermined, constant pressure. This ensures, for instance, that constant weights of packaged product are provided since packaging machines often operate on a timed basis.
A popular type of pump particularly suitable for use with food-related products is the reciprocating pump. One type of reciprocating pump utilizes a single piston and cylinder configuration. Typically, this pump will interact with a source of product such that a charge of product will be drawn into the cylinder on the piston's intake stroke and then discharged from the cylinder on the piston's discharge stroke. As is readily apparent, when using only a single piston it is impossible to maintain a constant flow of product under a uniform pressure since discharge only occurs during one-half of the piston's cycle.
In order to compensate for the lack of discharge over one-half of a typical single piston reciprocating pump's cycle, dual or multi-piston configurations have been devised. U.S. Pat. No. 3,108,318 to Miller et al. ("Miller I"), issued Oct. 29, 1963, is representative of this particular type of reciprocating pump. Miller I generally discloses a horizontal dual piston configuration in which one piston and its associated cylinder alternately reciprocate with a second piston and its associated cylinder to pump food products through a common discharge manifold. The disclosure indicates that both the first piston and its cylinder retract so that product may enter into a discharge chamber. After fully retracting, the first cylinder advances through the discharge chamber to trap a charge of product while the first piston stalls momentarily in its retracted position. After the first cylinder has properly seated against the end of the discharge chamber, the first piston advances through the cylinder to discharge the product therefrom. The second piston and its cylinder proceed under the same cycle, but the timing associated with the movement of the second piston and its cylinder is such that the second piston and cylinder are 180.degree. out-of-phase in relation to movement of the first piston and its cylinder. Consequently, when the first piston is discharging product, the second piston and cylinder are retracting to receive a charge of product in the discharge chamber. This dual piston configuration and the timing of their reciprocation thus produces a more uniform flow and pressure at discharge.
Alternately reciprocating dual piston pumps improve both the uniformity of the flow rate and the product pressure at discharge. However, pressure drops and decreases in flow rate discharge still occur since there is often a slight delay between the end of the discharge stroke of the first piston and the start of the discharge stroke of the second piston. In order to achieve consistent weights in packaged foods, it is important that such variations be reduced to acceptable levels. Consequently, numerous refinements of dual piston configurations have been proposed.
U.S. Pat. No. 3,456,285 to Miller et al. ("Miller II"), issued Jul. 22, 1969, discloses one type of alteration of a dual piston alternately reciprocating pump. Miller II generally discloses a horizontal dual piston pump in which both the pistons and their respective cylinders reciprocate as disclosed in Miller I discussed above. However, Miller II incorporates a number of additional features. For instance, the pistons and cylinders are positioned below a hopper which includes paddles to assist in forcing product down into the discharge area through which the cylinders pass to trap a charge of product. Furthermore, the cycles of the pistons overlap, which is achieved by driving the pistons with low and high pressure hydraulics. High pressure hydraulic fluid is used to drive the first piston through its cylinder to discharge product while the second piston and cylinder are being retracted. When fully retracted, the second cylinder is advanced through the discharge area to obtain a charge of product. After sealing of the charge by the second cylinder is completed, the second piston is advanced within the cylinder by low pressure hydraulic fluid. However, no product is discharged from the second cylinder since a valve positioned in the manifold connecting the outlets from the two cylinders only accepts flow from one cylinder at a time, that being the cylinder under the highest pressure. After the first piston completes its discharge stroke, high pressure hydraulic fluid is applied to the second piston to initiate its discharge stroke while the first piston and cylinder are retracted to complete the cycle which is thereafter repeated.
U.S. Pat. No. 4,191,309 to Alley et al., issued Mar. 4, 1980, discloses a product portioning or metering assembly used in combination with a pumping apparatus similar to that disclosed by Miller II. The disclosure of Alley et al. Illustrates, however, a number of variations in pump operations. For instance, the initial advancement of the first piston from its retracted position under the low pressure hydraulic fluid as the second piston is performing its discharge stroke under the high pressure hydraulic fluid is said to provide "precompression" which permits accurate metering of the product to be dispersed by removing air pockets therefrom prior to discharge. Presumably, this movement of the piston is termed as "precompression" since only low pressure hydraulic fluid is being applied to the first piston, and thus no product is discharged from its associated cylinder since discharge occurs only from the cylinder under the highest pressure. The precompression stroke continues until the low pressure hydraulic fluid is unable to overcome the increasing pressure within the cylinder as a result of compression of the product, at which point the first piston stalls. After the second piston completes its discharge stroke, high pressure hydraulic fluid is applied to the first piston, which remained in the stalled position by continually receiving the low pressure hydraulic fluid, to initiate its discharge stroke as the second piston and cylinder retract to repeat the above cycle.
U.S. Pat. No. 4,691,411, to Higashimoto, issued Sep. 8, 1987, discloses a dual piston alternately reciprocating pump which also incorporates a precompression stroke. This particular pump is a vertical pump which includes a hopper positioned above two feed cylinders, each of such cylinders having a reciprocating piston contained therein. A shutter plate is positioned between each cylinder and the hopper to essentially function as an inlet valve to the cylinders and the pistons move alternately within the cylinders through essentially intake and discharge strokes so as to provide a substantially uniform discharge of product. However, the pump also includes a precompression stroke to further refine the pressure variation at discharge. After a piston has reached bottom dead center ("BDC") and the shutter plate for the respective cylinder is closed, the piston advances through the cylinder on a precompression stroke which continued for a pre-determined time established by a time, regardless of the pressure attained in the cylinder during such precompression. After the lapse of the pre-determined time, further movement of the piston is terminated by the drive assembly, thereby completing the precompression stroke. Just prior to the time in which the other piston completes its discharge stroke, signal is sent to the drive assembly for the stalled piston to initiate its discharge stroke. After the lapse of a predetermined time, the other piston retracts on its intake stroke to complete the cycle. Although precompression is used by Higashimoto, its benefits are limited by the precompression stroke being defined by a pre-set time. Since different charges may possibly be obtained on each stroke when certain products are being pumped or with variations in pump speed, a time-dependent precompression stroke might not produce a consistent precompression pressure. This precompression pressure variation would adversely affect the pressure variation ar discharge.
U.S. Pat. No. 4,700,899 to Powers et al., issued Oct. 20, 1987, discloses another refinement of a dual piston alternately reciprocating pump. The general structure and operation or the pump disclosed by Powers et al. is similar to that disclosed by Miller I and II and Alley et al., utilizing two reciprocating pistons and cylinders. The heads of the pistons, however, are modified in that they incorporate a plurality of apertures through which a vacuum is drawn as the pistons are retracted on their respective intake strokes. This vacuum is maintained as the cylinder sleeves are propelled through the discharge chamber to allegedly assure full deaeration of the product.
Providing a pulsation free flow (i.e. free of deviation in discharge pressure), has of course not been limited to pumps used in the food industry. Reciprocating pumps have been designed to provide a pulsation free delivery of a liquid by a variety of methods. For instance, U.S. Pat. No. 4,359,312 to Funke et al., issued Nov. 16, 1982, allegedly provides a pulsation free delivery of a liquid from a dual piston alternately reciprocating pump by incorporating a feedback system which utilizes sensors positioned on the common discharge to ultimately adjust the speeds of the pistons, including adjustments which compensate when liquid is discharged simultaneously from both cylinders. U.S. Pat. No. 3,847,507, to Sakiyama et al., issued Nov. 12, 1974, discloses a feedback circuit for a single reciprocable piston (although reference is made to utilizing two pistons if a higher flow output is required) which uses a pressure sensor within the cylinder to activate a motor to move the piston within the cylinder to maintain a constant pressure on the discharged liquid. U.S. Pat. No. 1,723,874 to Lunge, issued Aug. 6, 1929; U.S. Pat. No. 2,010,377 to Sassen, issued Aug. 6, 1935; and U.S. Pat. No. 3,816,029 to Bowen et al., issued Jun. 11, 1974, each generally pertain to attempting to provide pulsation free delivery of a liquid by using specially designed cams to drive the pistons in a timed relation. The complexity of these types of apparatuses, as well as the results achieved, however, makes them somewhat undesirable for use in food-related applications.